Drilling systems are frequently used to provide cylindrical holes in metallic workpieces. The cutting or boring action of the drill system may be carried out by an elongated, substantially cylindrical drilling tool, such as a combination of a tool holder and a spade drill insert, which is selectively attached thereto. Such an arrangement may then be used in an application wherein one end of the tool holder is securely mounted in a driving apparatus, which rotates the holder about its longitudinal axis. At the opposite end of the elongated tool holder, the cutting insert engages the material to be cut. Alternatively, the workpiece may be made to rotate relative to the holder and cutting insert, such as in positioning the holder in the tail stock of a lathe or the like. Further, the tool and workpiece may be made to rotate relative to one another. The use of spade drill cutting inserts allows for quick changing of the insert upon wear of the cutting surfaces instead of the entire tool, and allows for one tool to be used for a variety of different boring applications by simply changing the insert and not the entire drill assembly.
Spade drill inserts are characterized in that they are generally flat having a pair of connection surfaces or faces on opposite sides of the insert that are parallel to each other and provide a register surface for the clamping arms of the holder. The leading end of the spade drill typically has a pair of straight, transverse cutting edges in the form of an obtuse V. The trailing end or base of the spade drill insert is typically planar and perpendicular to the pair of connection surfaces and engages the bottom of the holder slot. The sides across the width of the insert provide the margin and a clearance for the insert. The margin is cylindrically formed about the rotational axis of the spade drill and contacts the side of the hole during drilling operations.
One problem with prior art spade drill inserts is that the straight cutting edges do not efficiently form and/or transport chips during drilling operations. With the prior art spade drills, the chips are formed radially outward toward the outside diameter of the insert due to their straight cutting edge. Another problem with prior art inserts utilizing a corner clip clearance, the furthermost outside cutting edge is closer to the centerline of the lip thus being closer to a neutral or less positive rake geometry, especially when an oversized corner clip is utilized. This results in less efficient cutting at the outside diameter of the spade drill insert. A further problem with the prior art inserts is that the straight cutting edge chip formation is not tight which requires that the operational ranges of the tool to be narrowly defined for proper tool operation. In addition, the geometry of the straight cutting edges of prior art spade drills are apt to be susceptible to chatter primarily due to the chip formation energy along rotating symmetrical planes at a constant speed developing harmonics at various operating speeds depending on the material being cut and the natural frequency of the tool and system. This may result in the operator having to change the operating conditions of the drilling procedure to less than optimal ranges.